How can I add major geometry to a mesh in Sculpt mode?
Have you tried applying the multires modifier when you are happy with it? Then adding your primitives by joining (with all my desired mesh objects I wish to join selected I use Ctrl+J  to join them together)?

I enable dynotopo once I join my meshes to each other.

Depending on how rough/fine you want your sculpt to be at this point, will depend on how you set dynotopo's value. I personally like to use 5 or 6px.

I then lighten up the influence of my brush by setting it to near 0.0 with Shift+F and then dragging my mouse away from the center and click when I'm at my desired value. 

Then I lightly cover my transition points between my sculpt and my primitives with my sculpt brush (If this is slightly above 0.0 it will seem like it is not doing much, however you are adding mesh density).

Then I hold Shift and go back over and smooth the transitions.